Turbine stages, especially end stages, of conventional turbomachines are constructed in either an unshrouded or coupled manner. In this case, for vibration damping either a direct coupling between the individual turbine blades without an additional friction element, or an indirect coupling via a friction element, is possible.
In the case of a direct coupling, there is customarily a direct contact between adjacent blades, support wing connections and cover plate connections for example coming within this category. A direct coupling on the one hand leads to considerable stiffening of a blade ring and in most cases to low mistuning effects, while on the other hand only small damping effects can be achieved with the direct coupling.
All connecting elements, which lead only to a negligible coupling or to a negligible stiffening of the blade ring, are subsumed under an indirect coupling.
In the case of customary connecting concepts, which are used at present, for long turbine blades, especially in the case of long end-stage blades, a more or less direct coupling takes place. The disadvantages of a directly coupled connecting system, as mentioned in the introduction, lie in the damping behavior since the direct coupling during operation limits the relative movements between the blades and as a result less energy can be dissipated via friction damping. A further problem exists in the fact that long end-stage blades experience severe twisting when running up, as a result of which large displacements at the blade tip occur, which can lead to problems with the fastening of the connecting elements.
A connecting element for connecting two turbine blades at their blade tip is known from U.S. Pat. No. 4,401,411. For this, the turbine blades have in each case a cover plate which is arranged essentially at a right angle to the longitudinal extent of the blade and is provided with a through-hole which extends essentially parallel to the longitudinal direction of the blade. The connecting element has a pin which is formed complementary to this hole and with which the connecting element engages in the hole when connecting two adjacent turbine blades. The connecting element in this case is both to have a damping effect and to reduce twisting of the turbine blades around their blade longitudinal axis during operation of the turbine.
A coupling element for connecting two adjacent turbine blades is known from U.S. Pat. No. 4,257,743, wherein each coupling element has an essentially hemispherical negative recess in which the coupling element of the adjacent turbine blade engages with a hemispherical positive forming which is formed complementary to it.
Similar connecting elements for adjacent turbine blades are known from JP 11 01 3401 and JP 10 17 6501. The connecting elements which are known from these have a locking contour or mating locking contour formed complementary to it which are arranged in each case on a sealing element on a blade tip of the respective turbine blade. The sealing element therefore has a locking contour for one adjacent turbine blade, while it has a mating locking contour for the other adjacent turbine blade so that the sealing elements, which are essentially formed constructionally the same, of the turbine blades engage in each other without any problem.